1. Field of the Invention
The present invention relates to a discharge lamp lighting apparatus, and more particularly to a discharge lamp lighting apparatus to light a plurality of discharge lamps for use as a backlight in a liquid crystal display (LCD) apparatus.
2. Description of the Related Art
An LCD apparatus, which is a flat panel display apparatus, is used in various applications. Since a liquid crystal in the LCD apparatus does not emit light by itself, a lighting device is required in order to achieve a good display. A backlight device to light a liquid crystal panel from behind is among such lighting devices. In the backlight device, a cold cathode lamp is mainly used as a discharge lamp, and a discharge lamp lighting apparatus including an inverter to drive the cold cathode lamp is provided.
Recently, the LCD apparatus is becoming larger and larger for use in, for example, a large-screen TV, and therefore a number of discharge lamps are used in a backlight device in order to achieve sufficient screen brightness for the LCD apparatus. In such a backlight device, if variation exists in brightness of the discharge lamps, the display screen of the LCD apparatus incurs non-uniformity thus significantly degrading the display quality. So, not only high luminance of each discharge lamp but also brightness uniformity of all discharge lamps is required. Further, cost reduction of the discharge lamp lighting apparatus is requested along with the price reduction of the LCD apparatus.
The brightness variation of the discharge lamps can be prevented by equalizing lamp currents flowing respective discharge lamps for achieving a uniform brightness. Lamp currents can be equalized by a method such that transformers which are provided in a number equal to the number of the discharge lamps are individually controlled by respective control IC's. This approach, however, requires an increased number of components thus pushing up cost, which eventually results in an increased cost of the discharge lamp lighting apparatus.
Lamp currents can alternatively be equalized by providing balance coils, but this alternative approach requires a large number of balance coils for multiple discharge lamps, and the balance coils must be designed individually with different specifications because the values of currents flowing through the balance coils differ from one another depending on the places where the balance coils are disposed. Consequently, the number of components is increased pushing up the cost on the discharge lamp lighting apparatus.
A discharge lamp lighting apparatus as still another approach is proposed, in which inductance values are controlled by variable inductance elements, rather than balance coils, so as to control respective lamp currents and reduce the variation in brightness of the discharge lamps for uniform brightness over the display screen (refer to, for example, Japanese Patent Application Laid-Open No. H11-260580).
FIG. 3 is a block diagram for a circuitry of a discharge lamp lighting apparatus which is disclosed in the aforementioned Japanese Patent Application Laid-Open No. H11-260580, and in which two discharge lamps are provided.
Referring to FIG. 3, switching elements (FET's) 102 and 103 are connected in series between the positive and negative electrodes of a DC power supply 101, and the connection portion of the source terminal of the switching element 102 and the drain terminal of the switching element 103 is connected to the negative electrode of the DC power supply 101 via a series resonant circuit 120A which includes a capacitor 122a, and a coil 121a of an orthogonal transformer 121A constituting an variable inductance capable of controlling inductance values, and also via a series resonant circuit 120B which includes a capacitor 122a, and a coil 121a of an orthogonal transformer 121B constituting an variable inductance.
The connection portion of the coil 121a of the orthogonal transformer 121A and the capacitor 122a is connected to the negative electrode of the DC power supply 101 via a series circuit including a capacitor 110a, a discharge lamp 111a, and a current detecting resistor 123a of a control circuit 123A, and an output signal of the control circuit 123A is sent to a control coil 121b of the orthogonal transformer 121A.
The control circuit 123A supplies a control current to the control coil 121b of the orthogonal transformer 121A, and is arranged such that the connection portion of the discharge lamp 111a and the current detecting resistor 123a is connected to the inverting input terminal of an operation amplifying circuit 123c via a rectifier diode 123b, the connection portion of the rectifier diode 123b and the inverting input terminal of the operation amplifying circuit 123c is connected to the negative electrode of the DC power supply 101 via a smoothing capacitor 123d, the non-inverting terminal of the operation amplifying circuit 123c is connected to the negative electrode of the DC power supply 101 via a battery 123e having a reference voltage Vref to determine a reference value of a current of the discharge lamp 111a, and that the output terminal of the operation amplifying circuit 123c is connected to the negative electrode of the DC power supply 101 via the control coil 121b of the orthogonal transformer 121A.
The control circuit 123A functions to control the current of the discharge lamp 111a. Specifically, the control circuit 123A operates such that, when the current of the discharge lamp 111a is to be increased, the control current of the control coil 121b of the orthogonal transformer 121A is increased so as to decrease the inductance value of the coil 121a of the orthogonal transformer 121A thereby increasing the resonant frequency f0 of the series resonant circuit 120A thus decreasing the impedance of the series resonant circuit 120A at a driving frequency consequently resulting in an increase of a voltage generated across the both ends of the capacitor 122a, and such that, when the current of the discharge lamp 111a is to be decreased, the control current of the control coil 121b of the orthogonal transformer 121A is decreased so as to increase the inductance value of the coil 121a of the orthogonal transformer 121A thereby decreasing the resonant frequency f0 of the series resonant circuit 120A thus increasing the impedance of the series resonant circuit 120A at a driving frequency consequently resulting in a decrease of a voltage generated across the both terminals of the capacitor 122a. 
There is provided another circuit which includes another orthogonal transformer 121B, and which is constituted and functions identically with the above-described circuit including the orthogonal transformer 121A.
In the discharge lamp lighting apparatus shown in FIG. 3, a control circuit 104 fixedly sets a switching frequency of a control signal to be supplied to the switching elements 102 and 103 whereby the currents flowing through the discharge lamps 111a and 111b are controlled at a predetermined value without controlling the switching frequency, thus uniform brightness between the discharge lamps 111a and 111b is achieved without performing complicated frequency control at the control circuit 104.
A high voltage of about 1,500 to 2,500 V is required to turn on a cold cathode lamp, and a voltage of about 600 to 1,300 V must be applied to keep the cold cathode lamp lighted on. Accordingly, a power supply to supply such a high voltage is required in a discharge lamp lighting apparatus. Since the discharge lamp lighting apparatus shown in FIG. 3 is not provided with a step-up circuit, the DC power supply 101 has a circuitry to output a high voltage in order to duly turn on the discharge lamps 111a and 111b. 
Also, since the switching elements 102 and 103 to turn on the discharge lamps 111a and 111b, and the control circuit 104 to control the switching elements 102 and 103 are connected to the DC power supply 101 to output a high voltage, the switching elements 102 and 103 and the control circuit 104 must be composed of high withstand voltage materials which are expensive thus pushing up the cost of the components, and eventually the cost of the apparatus.
Further, in the discharge lamp lighting apparatus shown in FIG. 3, the capacitors 110a and 110b, which are current controlling capacitors (so-called “ballast capacitors”) to stabilize the lamp current of the discharge lamps 111a and 111b, are connected in series to the discharge lamps 111a and 111b, respectively, and a high voltage is applied to the capacitors 110a and 110b. Consequently, the capacitors 110a and 110b must also be composed of high withstand voltage materials, and since the current controlling capacitors must be provided in a number equal to the number of discharge lamps to be driven, the cost of the apparatus is pushed up definitely. Also, since a high voltage is applied to the capacitors 110a and 110b as described above, there is a problem also in terms of component safety.